I listened to this before I did any googling about it, and I highly recommend you do the same. The wikipedia page about the film on which this play is based consists of one line.
And that line is a spoiler.
A little background. This radio play is an adaptation of sorts of a TV play (released in 1964) created by Nigel Kneale, the genius behind Quatermass, The Stone Tape, and The Year of the Sex Olympics (look it up). Mark Gatiss tried to get it made previously (see his obituary of Kneale (spoilers!) but it came to naught. It's not a surprise then to see him taking the role of Gideon Cobb in Toby Hadoke's new adaptation.
It is 1768. Tetsy, a country girl, (Susan Wokoma), has experienced strange goings-on in a nearby wood, and her local squire is investigating. The squire (Adrian Scarborough) has something of an interest in the scientific method, and is applying a whole range of techniques to the problem. This includes the particularly novel idea of "fluids", which can be collected from lighting or living creatures in some manner. It's one of the more convincing versions of early electricity experiments I've seen.
The squire's wife, Lavinia (Hattie Morahan), when visiting London, happened to run into a philosopher, who has spent his life hunting for the truth, and brought him back with her in the hopes that he can assist. (And possibly also in the hopes she can sleep with him.) This philosopher (played by Gatiss) has performed experiments of his own, breaking down the mind of Jethro (Colin McFarlane), his black slave, and then rebuilding it to be a constant searcher of truth.
That sets up the conflict between the three parties. Gideon is a forceful figure, who is certain that all of the problems of man can be solved by thinking about them from a logical standpoint, and no wider investigation is needed. The squire is querulous, but open minded to a fault, willing to try everything, even if it seems like alchemy. And Jethro is determined to decide between them, being better educated and informed than most of the population, despite occupying an underprivileged position because of his race. He's reminiscent of the "Fair Witnesses" from Stranger in a Strange Land, who are so obsessed with the truth that the act of lying is impossible for them.
All of this is set against the events occurring in the wood, spooky phantasms that appear on the fabled site of a famous defeat for Boudicea's army. They each have their own opinion and approach, and the fights between them are the majority of the play. Gatiss is excellent as the arrogant, supercilious philosopher, and Scarborough's nervous but earnest scientist is a wonderful balance. Their back and forth is a nice exploration of different viewpoints on investigation, but I've got to give McFarlane points for his understated performance that does a lot with very little for most of the work.
The various arguments and protestations are all overshadowed by the climax, which rather smacked me in the face on a first listen, but is more foreshadowed on a repeat. Some of the intended political points come across somewhat heavy-handed on occasion, but given the inspiration, that's understandable. It's a morality play of sorts, but a brilliant one. Give it a listen (and avoid spoilers!)